(1)    For example:

use strict;
my @a = ('A', 'B', [1, 2, ['C', 'D'], ['X', [11, 13, 15], 'Y'], [30]], [], 400);
print DepthFirstTraversal(@a);
exit 0;

sub DepthFirstTraversal {
    my $list = @_;
    my $result = "";
    foreach (@_) {
        if (ref $_ eq 'ARRAY') {
        $result .= DepthFirstTraversal(@{$_});
        }
        else {
            $result .= "\"$_\"  ";
        }
    }
    $result;
}

(2)    For example,

perl -ne "print $_ unless $w{$_}; $w{$_}++;" somefile.txt >somefile_out.txt

(3)    For example,

use strict;

# h3q3.pl
# by Kwok-Bun Yue
# This program reads a text file of discussion board
# header and print out part of the HTML code for
# listing the message headers in a tree structure.
#
my $HEADER_FILE = "h3q3dat.txt";
my $SEPARATOR = '!!';
# array of children nodes (arrays)

# Store information of all children nodes.  For examples, if nodes 3, 5 and
# 7 are children of node #1, then
# $children[1] = ["3!!name of 3!!email of 3!!title of 3",
#     "5!!name of 5!!email of 5!!title of 5",
#     "7!!name of 7!!email of 7!!title of 7",
my @children = ();
# Read data file to populate @children.
open DATA, "<$HEADER_FILE" || die "Can't open file $HEADER_FILE";
while ($_ = <DATA>) {
 chomp;
 my ($id, $parent, $name, $email, $title) = split /$SEPARATOR/;

 if ($children[$parent]) {
  push @{$children[$parent]}, "$id$SEPARATOR$name$SEPARATOR$email$SEPARATOR$title";
 }
 else {
  $children[$parent] = ["$id$SEPARATOR$name$SEPARATOR$email$SEPARATOR$title"];
 }
}
close DATA;

if ($children[0]) {
 # Have at least one node.
 printTree(0);
}

# Print the current subtree with the given root node recursively.
sub printTree {
 my $rootNode = shift;
 print "<ul>\n";
  for (my $i=0; $i< @{$children[$rootNode]}; $i++) {
   my ($child, $name, $email, $title) = split /$SEPARATOR/, ${$children[$rootNode]}[$i];
   print "<li><a href=\"mailto:$email\">$name</a>: $title</li>\n";
   printTree($child) if $children[$child];
  }
 print "</ul>\n";
}
exit 0;